Sounds to me like they don't have Andrew Garfield signed for a fourth movie yet, and they're hedging their bets. This could even be part of a negotiating tactic to show they're willing to go on without Andrew if he asks for too much money.
I would imagine that Andrew has to be either very naive or delusional if he doesn't already know that he's not indispensable to the franchise. They've already done one trilogy with another actor and its well known that Toby was almost replaced by Jake Gylenhall for the second movie, when he tried to play hardball. Anyway, Andrew, like Toby, will be well into his 30s by the time of any 4th movie and the series will probably be due an infusion of new blood anyway.